I'm looking to project certain video images on the side of buildings etc for a project (at least 6ft high/11 feet wide I would have thought). Does anyone have any experience of this and recommend a certain projector to achieve this?

The larger the projectors ratings, the brighter it's output. But that's only one factor to consider.

As with all projection anywhere, brightness rests on three things, the first being the actual brightness of the projector... the others are how far away the projector will be positioned from the screen surface (inverse square principal of light as waves) and what the ambient light conditions will be that surround that projection surface.

Those two issues will either help or hurt your achieving a particular brightness at the screen surface.

Here in the US, most of the really bright, but still portable projectors I've worked with are from BARCO - with quite a few offerings putting out a 10,000 ANSI Lumen or even brighter image.

Other manufacturers are surely out there with similar equipment.

Also remember that really, really bright, often requires a really robust power draw. So you might benefit from the help of a qualified electrician if you're going to have to tap into serious power.

Finally, it's pretty common to STACK projectors running the same content and register the images carefully to each other if you need more brightness than a single projector can output.